Journal of Surgical Research 148, 230 –237 (2008) doi:10.1016/j.jss.2007.08.003
A Reproducible Porcine ePTFE Arterial Bypass Model for Neointimal Hyperplasia 1 Muneera R. Kapadia, M.D., Oliver O. Aalami, M.D., Samer F. Najjar, M.D., Qun Jiang, M.D., Jozef Murar, B.A., Brian Lyle, B.A., Jason W. Eng, Bonnie Kane, B.S., Timothy Carroll, Ph.D., Patricia M. Cahill, D.V.M., and Melina R. Kibbe, M.D.2 Division of Vascular Surgery, Northwestern University Feinberg School of Medicine, Chicago, Illinois Submitted for publication May 29, 2007
Background. Late failure of prosthetic vascular bypass grafting using expanded polytetrafluoroethylene (ePTFE) is secondary to the development of neointimal hyperplasia, most commonly at the distal anastomosis. To develop therapies that can improve upon current prosthetic vascular bypass grafting, a large animal model of prosthetic bypass grafting that results in reproducible neointimal hyperplasia is necessary. Methods. We performed bilateral end-to-side carotid artery bypasses with 6 mm ePTFE in a porcine model (n ⴝ 11). We studied graft patency using magnetic resonance angiography (MRA, 3 wk), duplex ultrasonography (4 wk), and digital-subtraction contrast angiography (4 wk). Animals were sacrificed at 4 wk and morphometric analysis was performed. Results. Of the 11 animals that underwent surgery, one pig died from respiratory compromise; of the remaining 10, graft patency was 90% at 4 wk. Peak systolic and end diastolic velocities were established for this model using ultrasonography. MRA, ultrasonography, and angiography confirmed graft patency and were complimentary tools to evaluate the grafts. Development of neointimal hyperplasia was reproducible at 4 wk in both the proximal and distal anastomoses (2.5 to 3 mm 2) of the ePTFE bypass grafts. Conclusion. We developed a reproducible porcine ePTFE carotid artery bypass model for studying neointimal hyperplasia. Not only does this model allow for the manipulation and evaluation of potential therapies, but patency and neointimal hyperplasia can be 1 Muneera R. Kapadia and Oliver A. Aalami share co-first authorship. 2 To whom correspondence and reprint requests should be addressed at Division of Vascular Surgery, Northwestern University Feinberg School of Medicine, Galter 10-105, 201 E. Huron Street, Chicago, IL 60611. E-mail:
[email protected].
0022-4804/08 $34.00 © 2008 Elsevier Inc. All rights reserved.
easily evaluated by traditional means, such as MRA, ultrasonography, and angiography. This preclinical model is ideal for evaluation of novel therapies in vivo designed to inhibit neointimal hyperplasia following arterial reconstruction with prosthetic bypass grafting. © 2008 Elsevier Inc. All rights reserved. Key Words: restenosis; neointimal hyperplasia; porcine; ePTFE; peripheral arterial disease; bypass. INTRODUCTION
Failure of arterial revascularization secondary to neointimal hyperplasia remains a clinical problem associated with significant morbidity and mortality. All vascular procedures result in endothelial cell damage, platelet and leukocyte adherence to the site of injury, and vascular smooth muscle cell proliferation and migration, which result in neointimal hyperplasia and arterial restenosis [1– 4]. In patients requiring vascular bypass grafting with prosthetic material, neointimal hyperplasia develops aggressively, especially at the distal anastomosis. In fact, according to Veith et al., only 54% of below-knee bypass grafts with expanded polytetrafluoroethylene (ePTFE) are patent at 4 y [5]. More recently, Stonebridge et al. reported 2-y patency rates for uncuffed femoral to below-knee ePTFE bypass grafts to be only 29% [6]. The exaggerated neointimal hyperplasia forms secondary to graft thrombogenicity due to lack of endothelium, low flow states when used in diameters less then 6 mm, and compliance mismatch between the prosthetic material and the native vessel [7, 8]. Because of these significant problems with prosthetic grafts, researchers have devoted significant time and resources to developing alternate materials for vascular bypass grafting. Large animal models that
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tend to have similar anatomy and physiology to humans are instrumental, especially when evaluating new potential therapies [9]. For the study of neointimal hyperplasia, rabbit, canine, primate, and porcine models have been described. While each of these models has advantages, each also has significant limitations. Therefore, we sought to develop a reproducible large animal model of prosthetic arterial bypass grafting to reliably evaluate therapies designed to reduce the development of neointimal hyperplasia following prosthetic bypass grafting. We designed and evaluated a porcine carotid artery ePTFE bypass graft model. The benefits of our model include reproducibility, simple exposure without the need to enter a major body cavity, and the ability for accurate noninvasive monitoring using duplex ultrasonography or magnetic resonance angiography (MRA). METHODS Presurgical Care and Anesthesia All animal procedures were performed by use of aseptic technique in accordance with the Northwestern University Animal Care and Use Committee. Domestic juvenile castrated male YorkshireLandrace pigs (Oak Hill Genetics, Ewing, IL) weighing 25 to 30 kg were used. Animals received antibiotic prophylaxis with one dose of cefazolin (25 mg/kg intramuscular [i.m.]) preoperatively and postoperatively. Aspirin (325 mg oral [p.o.]) was administered daily starting 5 d prior to surgery and continued throughout the entire postoperative course. On the day of surgery, the animals also received an aspirin suppository (325 mg). Preop analgesia and sedation included buprenorphine (0.01 mg/kg i.m.), acepromazine (0.15 mg/kg i.m.), ketamine (20 mg/kg i.m.), and atropine (0.05 mg/kg i.m.). After intubation, anesthesia was maintained with inhaled isoflurane (0.5% to 2.0%) delivered with 100% oxygen. Temperature, heart rate, respiratory rate, and oxygen saturation were monitored continuously and recorded every 15 min throughout the procedure.
Surgical Procedure Animals were placed in the supine position, and their necks were shaved, then prepped with betadine and alcohol (70%). Pigs received bilateral ePTFE bypass grafts (n ⫽ 11) (Fig. 1). Through a midline neck incision, both right and left common carotid arteries (CCA) were exposed. Following heparin (150 U/kg intravenous [i.v.]) administration, the right CCA was occluded proximally and in the midsection with noncrushing vascular clamps. A longitudinal arteriotomy was made, and a 6 cm length of ePTFE graft (6 mm thin wall stretch ePTFE; Gore, Flagstaff, AZ) was anastomosed in an end-toside fashion with running 6-0 polypropylene suture. Care was taken to ensure that the ePTFE graft was anastomosed at a 45° angle with respect to the native artery, thereby dictating a standard arteriotomy length. Prior to completion of the proximal anastomosis, the native vessel was flushed and irrigated with heparinized saline (2000 U heparin per 1 L normal saline). After completion of the proximal anastomosis, flow was restored in the CCA for 5 min while the graft was clamped near the anastomosis. Next, the right CCA was occluded distally and in the mid-section with noncrushing vascular clamps. The distal anastomosis was created in a manner similar to the proximal anastomosis. Just prior to its completion, the graft and native artery were vigorously flushed with heparinized saline. Once the distal anastomosis was completed, the distal arterial clamp was removed, restoring blood flow to the CCA through the ePTFE graft.
FIG. 1. Intraoperative photographs of (A) the right ePTFE carotid artery bypass graft and (B) bilateral ePTFE carotid artery bypass grafts. Arrows indicate ePTFE grafts. (Color version of figure is available online.)
Sub-adventitial papaverine injections (30 mg/mL) were used to prevent arterial spasm, especially in the distal CCA. The mid-section vascular clamp was replaced with double ligation using 2-0 silk suture to simulate an occlusion. After completion of the right bypass graft, a second dose of heparin (75 U/kg i.v.) was administered, and the left bypass graft was created in a similar manner. After both bypass grafts were completed, meticulous hemostasis was achieved, and the incision was closed in multiple layers using absorbable suture. Animals were monitored until awake, alert, and sternal. Postoperative analgesia consisted of buprenex (0.01 mg/kg i.m.) given every 12 h for the first 48 h postoperatively.
Magnetic Resonance Angiography Magnetic resonance angiography was performed to evaluate graft patency 3 wk postoperatively. After sedation with acepromazine (0.15 mg/kg), atropine (0.05 mg/kg), and ketamine (20 mg/kg), MRA was performed using a time-resolved T1-weighted gradient echo pulse sequence. A time-series of 3D contrast-enhanced images were acquired with a gadolinium-based contrast agent (0.1 mmol/kg i.v.; Magnevist, Berlex, Princeton, NJ). Noncontrast angiograms were also performed using a time-of-flight imaging protocol.
Duplex Ultrasonography and Angiography Four weeks postoperatively, just prior to sacrifice, both ultrasonography and contrast angiography were conducted. Following exposure of both the right and left CCA and bypass grafts, intraop-
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erative duplex ultrasonography was performed, obtaining B-mode images and Doppler velocity measurements including peak systolic velocity (PSV) and end diastolic velocity (EDV). For each side, measurements were obtained at the proximal CCA, proximal anastomosis, proximal graft, mid-graft, distal graft, distal anastomosis, and distal CCA. A significant stenosis was defined as PSV greater than two times the normal inflow artery velocity. Next, digital subtraction contrast angiography was performed percutaneously by accessing the common femoral artery and placing a 5 F introducer sheath. Using a 0.035-in. J-wire, a 5F multisidedhole catheter was advanced into the aortic arch under fluoroscopy. An arch angiogram was obtained with contrast injection (20 mL; Omnipaque, Amersham, Piscataway, NJ) through the catheter. Each CCA was selected using an angled guide catheter and the J-wire, and angiograms were obtained of both CCA and ePTFE grafts using 10 ml of contrast media. Following angiogram completion, the animals were euthanized with pentobarbital (72 mg/kg).
Tissue Processing The ePTFE bypass grafts and native artery extending 2 cm from each anastomosis were harvested en bloc and underwent ex vivo perfusion-fixation in formalin overnight. Next, the samples were dehydrated with a graded ethanol series, cut into 1 cm sections, and placed in plastic tissue cassettes (Tissue-Tek, Hatfield, PA). The samples were then embedded in paraffin, and blocks were cut into 5 m sections.
Tissue Staining and Morphometric Analysis Sections were examined histologically for evidence of neointimal hyperplasia using routine hematoxylin and eosin staining. Digital images were collected with light microscopy using an Olympus BHT microscope (Melville, NY). Five equally-spaced sections throughout each proximal and distal anastomoses were analyzed for each bypass graft (Fig. 2). Neointimal hyperplasia (area in millimeters) under the ePTFE graft was assessed using ImageJ software (National Institutes of Health, Bethesda, MD). To accurately compare the degree of neointimal hyperplasia from each anastomosis, the angle of embedding and cutting the block with respect to the angle of the ePTFE to the native artery was accounted for and controlled so that each block was similar, as changes to the block orientation can artificially underestimate or overestimate the degree of neointimal hyperplasia.
FIG. 2. Diagram depicting the five equally-spaced sections throughout the ePTFE-arterial anastomosis that were used for morphometric analysis. (Color version of figure is available online.)
FIG. 3. Magnetic resonance angiography of the bilateral ePTFE carotid artery bypass grafts from the (A) anterior-posterior view and (B) right anterior oblique view. Arrows indicate proximal anastomoses, and arrowheads indicate distal anastomoses.
Statistical Analysis Results are expressed as mean ⫾ standard error of the mean. Differences between multiple groups were analyzed by use of oneway analysis of variance with the Student-Newman-Keuls post hoc test for all pair wise comparisons. Differences between two groups were analyzed with the Student’s t-test (SigmaStat; SPSS, Chicago, IL). Statistical significance was assumed when P ⬍ 0.05.
RESULTS Surgery and Outcomes
Juvenile male Yorkshire-Landrace pigs underwent carotid artery bypass with ePTFE grafts (n ⫽ 11) (Fig. 1). Early on, the intraoperative acute thrombosis rate was 50%. These occurred in the beginning of the series and caused us to modify our intraoperative anticoagulation regimen. Initially, we were administering only one dose of heparin (100 U/kg i.v.). After experiencing intraoperative graft thromboses, we began administering two heparin doses; 150 U/kg was administered before the first graft and an additional 75 U/kg prior to the second graft. This dramatically reduced the occurrence of intraoperative graft thrombosis for all subsequent operations. All grafts were patent at the completion of the procedure. One animal developed malignant hyperthermia at the completion of the procedure, however recovered without sequelae. One animal died immediately postoperatively secondary to respiratory compromise. The postoperative course for each animal was uncomplicated. No late neurological complications were observed. At sacrifice, 10% of the grafts were thrombosed (2 out of 20). Several animals had minor wound infections; however, there were no septic complications.
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FIG. 4. Characteristic ultrasound waveform and B-mode images of the (A) proximal anastomosis (PA), (B) mid-graft (MG), and (C) distal anastomosis (DA). (D) Mean peak systolic velocity (PSV) and mean end diastolic velocity (EDV) measurements throughout the grafts. PArt ⫽ proximal artery; PG ⫽ proximal graft; DG ⫽ distal graft; DArt ⫽ distal artery. (Color version of figure is available online.)
FIG. 5. Contrast angiography of the right ePTFE carotid artery bypass graft from the right anterior oblique (RAO) view. Arrow indicates proximal anastomosis, and arrowhead indicates distal anastomosis.
Magnetic Resonance Angiography
difference within the PSV or EDV for the different areas of the graft analyzed. Thus, we have established baseline PSV and EDV in this porcine ePTFE carotid artery bypass model.
MRA was performed 3 wk following the initial procedure to assess graft patency (Fig. 3). Reconstructed images demonstrated that the ePTFE grafts were patent. Minimal stenosis was present at either the proximal or distal anastomosis. Ultrasonography
On the day of sacrifice, intraoperative duplex ultrasonography was performed (Fig. 4). The PSV and EDV for each segment are shown in Table 1. Note that while there is a more significant variation in the PSV compared with the EDV, there is no statistically significant TABLE 1 Duplex Ultrasonography Velocity Measurements
Section
Peak systolic velocity (m/s) mean ⫾ SE
End diastolic velocity (m/s) mean ⫾ SE
Proximal artery (PArt) Proximal anastomosis (PA) Proximal graft (PG) Mid graft (MG) Distal graft (DG) Distal anastomosis (DA) Distal artery (DArt)
0.583 ⫾ 0.178 0.677 ⫾ 0.285 0.548 ⫾ 0.207 0.444 ⫾ 0.158 0.515 ⫾ 0.192 0.590 ⫾ 0.200 0.732 ⫾ 0.232
0.128 ⫾ 0.064 0.135 ⫾ 0.047 0.103 ⫾ 0.044 0.078 ⫾ 0.032 0.107 ⫾ 0.053 0.095 ⫾ 0.028 0.114 ⫾ 0.049
Note. P ⫽ NS for all sections within each group.
Contrast Angiography
Following ultrasonography and just prior to sacrifice, digital subtraction contrast angiography was performed (Fig. 5). Angiography is the gold standard when evaluating vessel and graft patency. Results were concurrent with the MRA and ultrasonography data. Neointimal Hyperplasia
Both the proximal and distal anastomoses of each ePTFE graft were analyzed and quantified for neointimal hyperplasia (Fig. 6A). The neointimal hyperplasia (Fig. 6B) at the proximal anastomosis was 2.93 ⫾ 0.24 mm 2 and the distal anastomosis was 2.64 ⫾ 0.26 mm 2; there was no statistically significant difference between these measurements (P ⫽ NS). Importantly, the standard error for each of these is 10% or less than the mean value for either the proximal or distal anastomosis, indicating that our model is reproducible. DISCUSSION
Neointimal hyperplasia and restenosis following arterial bypass grafting with prosthetic material often results in treatment failure and significant morbidity and mortality. Therefore, it is critical to study this
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FIG. 6. (A) Representative hematoxylin and eosin-stained crosssections from the proximal artery, proximal anastomosis (PA), distal anastomosis (DA), and distal artery. (B) Morphometric analysis of PA and DA conducted on five equally-spaced sections throughout each anastomosis (n ⫽ 10 pigs). NS ⫽ not significant. (Color version of figure is available online.)
problem and important to have animal models that allow us to do so. In this manuscript, we present a porcine carotid artery ePTFE bypass graft model for the study of neointimal hyperplasia. This model simulates conditions seen in human arterial bypass procedures with prosthetic material. It is reproducible as evidenced by the consistent development of neointimal hyperplasia seen in all of our pigs. The surgical exposure is simple and avoids violation of any major body cavities. The carotid arteries and bypass grafts are easily evaluated with MRA, duplex ultrasonography, and digital subtraction contrast angiography. This model is ideally suited to study novel therapies aimed at reducing neointimal hyperplasia following arterial bypass grafting procedures with prosthetic materials. Animal models have been instrumental in the study of neointimal hyperplasia. Small animal (e.g., mouse and rat) and rabbit models are inexpensive and provide
a good initial in vivo environment to examine potential therapies. However, to determine whether a therapy will be effective in patients, it should be evaluated in a large animal model. Large animal models more closely resemble human physiology and anatomy, and are most predictive of results in humans [9]. This point has been illustrated recently with the E2F decoy trials. Initially, the E2F decoy therapy, an oligodeoxynucleotide competitive inhibitor of the E2F transcription factor that results in cell-cycle blockade was shown to be quite effective in rat and rabbit models of arterial injury and vein bypass grafting [10, 11]. Before additional large animal data were collected, this therapy was initiated in patients with the PREVENT trial, demonstrating safety and feasibility [12]. Unfortunately, in the PREVENT III and IV trials, no significant improvement in outcome was observed in patients undergoing lower extremity arterial reconstruction or coronary artery bypass grafting, respectively [13, 14]. This example emphasizes the importance of evaluating potential therapies in more clinically relevant animal models prior to administration to patients. Many investigators have used nonhuman primate, canine, and porcine models to study therapies for the inhibition of neointimal hyperplasia following arterial interventions; however, each of these species has its own unique advantages and disadvantages. Nonhuman primates are most anatomically and physiologically similar to humans, but are often cost prohibitive. Additionally, they may not offer sufficiently better results than other large animal models to justify their use. Dogs are less costly and easier to work with than nonhuman primates, but their unpredictable hypercoagulability and potent fibrinolytic system can be problematic in vascular research [15]. Porcine models, while hypercoagulable, are less expensive and do not present the ethical dilemmas associated with nonhuman primates [15]. Furthermore, when compared with canine models, porcine models of arterial injury have been shown to be more predictive of results in humans [9, 16]. Taking these considerations into account, we chose to focus on porcine models. Currently described porcine restenosis models with prosthetic materials include variations of arteriovenous and arterio-arterial bypass grafts [17–23]. The arterio-venous bypass graft represents a different pathophysiologic process that results in neointimal hyperplasia when compared to arterial reconstruction [24]. Hemodynamic forces are thought to play an important role in this differential response. Arterial bypass grafts are pulsatile; combined with compliance mismatch, particularly with ePTFE grafts, neointimal hyperplasia typically forms at the heel, toe, and a short segment of the floor of the distal anastomosis [24]. In contrast, arterio-venous fistulas have blood flows 5 to 10 times higher than arterial bypass grafts and, as a
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consequence, experience much more turbulence and higher wall shear stress. The pattern of neointimal hyperplasia in arterio-venous fistulas is at the heel, toe, and a long segment of the floor of the distal vein [24]. Therefore, arterio-venous bypass grafts are not an ideal model for studying neointimal hyperplasia following arterial bypass grafting. These two models should be considered as two distinctly different models evaluating two entirely different processes. It should also be mentioned that there are ePTFE interposition graft models that have been described [17, 18]. For the most part, these models are also inappropriate, as arterial interposition grafts are rarely placed in patients in the lower extremity. The end-to-side anastomosis is the most commonly used anastomosis clinically. There are some established porcine prosthetic arterioarterial bypass models [19, 22]. However, when we began to consider models of arterial bypass grafting, we considered the following possibilities: aorto-common iliac, common iliac-common iliac, common iliac-external iliac, or common iliac-femoral artery bypass grafting. In the aorto-iliac bypass model, because the bypass graft arises from the aorta, a high flow conduit, it does not mimic the conditions normally seen in peripheral vascular occlusive disease. Additionally, both right and left proximal anastomoses arise from the same vessel, namely the aorta, making it difficult to evaluate neointimal hyperplasia at the proximal anastomosis. In the common iliac-common iliac artery bypass model, the proximal anastomosis arises from the common iliac artery, which more closely resembles conditions seen with an arterial bypass grafting. However, because the common iliac artery is fairly short, the proximal anastomosis must be placed close to the aortic bifurcation, and the distal anastomosis must be placed close to the iliac bifurcation, leaving little artery to analyze on either side of the graft. The common iliac-external iliac or common iliac-femoral artery bypass presents additional technical challenges because the porcine femoral arteries are small in diameter, predisposing them to thrombosis. Furthermore, because of the small size of the external iliac and common femoral arteries compared with the common iliac artery, significant size mismatch exists between the native artery and the 6 mm ePTFE conduits. Additionally, each of these models requires dissection in the abdominal cavity, which can be associated with genitourinary and gastrointestinal complications. By comparison, our carotid-carotid artery bypass model has several advantages. The dissection is simple, and does not violate a major body cavity. The superficial position of the CCA makes it ideal for noninvasive monitoring with duplex ultrasonography. This modality allows not only for surveillance of graft patency, but also for graft stenosis via measurement of flow velocities. In addition, the CCA is straight and has
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a diameter appropriate for a 6 mm ePTFE graft, making the procedure technically straightforward and reproducible, which results in consistent development of neointimal hyperplasia. Finally, because the porcine CCA is long, several centimeters of artery can be harvested on either side of the graft for analysis. We compared our results with those of other groups performing arterial-ePTFE bypasses with end-to-side anastomoses. Mellander et al. used 4 mm ePTFE grafts from the common iliac to external iliac artery [19]. This study used conventional pigs (similar to our study) but analyzed sections only in the mid-portion of the distal anastomosis. At 30 d, neointimal hyperplasia was measured to be approximately 2 ⫾ 1 mm 2. While there are differences in ePTFE diameter (we used 6 mm ePTFE), the area of neointimal hyperplasia in our model, 2.5 to 3.0 mm 2 (comparing both proximal and distal anastomosis data), was in a similar range. Another group, Cagiannos et al., performed end-to-side anastomosis with 6 mm ePTFE for aorto-iliac bypass grafting in mongrel pigs [22]. The authors calculated neointimal thickness in arbitrary units, and only evaluated neointimal hyperplasia at the heel of the distal anastomosis after the samples were cut open longitudinally. Furthermore, no histology images were published with that manuscript, making comparisons even more difficult. In our manuscript, we examined the area of neointimal hyperplasia throughout the entire anastomosis of both the proximal and distal anastomoses, ensuring that our results accurately represent the amount of neointimal hyperplasia. The anastomoses were also processed intact, not cut open, thereby more accurately depicting what occurs in vivo. While our model has several benefits, there are some limitations that must be considered. The CCA as a model of neointimal hyperplasia development differs from the lower extremity in that comparatively, the outflow resistance is lower. The lower extremity is a high resistance system and, as such, the neointimal hyperplasia may develop in a more aggressive manner. Additionally, the main surgical complication that we experienced from bilateral carotid artery exposure was respiratory distress. This may be due to tracheal edema or, more likely, vagus or recurrent laryngeal nerve injury during surgery. Minimal dissection and careful arterial clamp placement during the procedure is imperative to avoid this consequence. Furthermore, administration of antibiotics on arrival to the facility for 2 d to combat shipping stress has significantly reduced the incidence of postoperative respiratory distress. This is in addition to the routine administration of perioperative antibiotics. Lastly, it should be pointed out that the intention of this study was to develop a reproducible prosthetic bypass grafting model of neointimal hyperplasia, not hemodynamically significant stenoses. Since these grafts were analyzed at 4 wk, one
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would not expect the development of hemodynamically significant lesions. However, if the grafts were observed over longer time periods, such as 3 or 6 mo, hemodynamically significant lesions would likely have developed. Routine use of the noninvasive imaging modalities assessed in this study (i.e., ultrasound, MRA, etc.) would be of great benefit at these later time points. During the early part of this study, we encountered significant problems with intraoperative graft thrombosis, which forced us to modify our protocols. In our experience, perioperative antiplatelet therapy, intraoperative heparin, and minimizing arterial spasm are critical to decrease early graft thrombosis. Our practice evolved from administering one dose of heparin (100 U/kg) to two doses: 150 U/kg prior to the first bypass and 75 U/kg prior to the second bypass. This change had a significant impact on the intraoperative graft thrombosis rate. We also started daily aspirin 5 d prior to surgery and throughout the postoperative course. Recently, we added a half-dose of clopidogrel (37.5 mg) on the day of surgery to help further prevent graft thrombosis. Lastly, arterial spasm appears to be prominent with porcine arteries. Over the course of these experiments, we developed a process of bathing the arterial segment in diluted papaverine (60 mg/500 mL normal saline) between each anastomosis. After completion of an anastomosis, we injected concentrated papaverine (30 mg/mL) into the adventitia of the native artery just after restoration of flow. We also added 1/2-in. nitropaste to the ear at the initiation of the surgery. We noticed that all of these maneuvers significantly reduced the incidence of intra- and postoperative graft thrombosis. The benefits of our porcine model are its arterioarterial design, simple exposure that does not violate a body cavity, the ability to monitor graft patency with multiple modalities and, most importantly, reproducibility. With high rates of prosthetic graft failure, an exciting area of research is the development of new materials to improve graft patency. Therefore, it is imperative that researchers have appropriate animal models. Our porcine carotid artery ePTFE bypass model is well-suited to study the efficacy of novel bypass graft therapies to inhibit neointimal hyperplasia.
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ACKNOWLEDGMENTS This work was supported in part by funding from a State of Illinois Excellence in Academic Medicine grant, a Northwestern Memorial Foundation Research and Education grant, a Northwestern Memorial Hospital Salerno Educational grant, a Department of Veterans Affairs Merit Review grant, and by the generosity of Mrs. Hilda Rosenbloom. In addition, the authors express their thanks to the Northwestern University Institute for Bionanotechnology in Medicine, the Feinberg Cardiovascular Research Institute, and to Lynnette Dangerfield for her administrative support.
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